This invention relates to a pump mechanism for ejecting liquid which can eject a fixed amount of a given liquid, such as a liquid medicine, at a time in the form of a spray or jet.
Containers for holding a liquid medicine for nose or throat treatment, for instance, are usually provided with a built-in pump mechanism for ejecting the liquid medicine on affected parts of a human body.
As illustrated in FIG. 5, a conventional pump mechanism comprises a stationary cylinder 1 which is mounted at a mouth of a liquid medicine container, a first piston 2 and a second piston 3, both incorporated inside the stationary cylinder 1 in a coaxial configuration. The first piston 2 is slidably installed with its lower bell-shaped portion 2b fitted in a large-diameter portion 1a of the stationary cylinder 1 provided close to its upper end, while the second piston 3 is installed with its upward-directed funnel-shaped portion 3b fitted into a small-diameter portion 1b of the stationary cylinder 1. As the second piston 3 moves up and down, its upper sloping surface 3a closes and opens a stepped axial hole 2c in the first piston 2 from underneath. The second piston 3 is fitted with a tubular nonreturn valve 3d formed of an elastic material, such as rubber, which can close an axial hole 3c connected to an unillustrated intake port from outside.
As the first piston 2 is depressed by pushing an unillustrated operating nozzle unit, a mass of liquid medicine contained in a metering chamber A, which is formed between the bell-shaped portion 2b and funnel-shaped portion 3b, is forced into the axial hole 2c and sprayed through a nozzle chip mounted at an end of the first piston 2 in the direction of an arrow Ka as illustrated in FIG. 5. More specifically, when the first piston 2 is pressed down, the internal pressure of the metering chamber A increases and the second piston 3 is forced downward against an unillustrated spring. Consequently, the axial hole 2c is opened, allowing the liquid medicine to be delivered upward and sprayed through the nozzle chip. In this spraying operation, the liquid medicine contained in the metering chamber A flows into a lower space of the axial hole 2c through slits 2d1 made in a cylindrical guide 2d which extends downward from a lower part inside the bell-shaped portion 2b.
When a pressuring force applied to the first piston 2 is removed, the first piston 2 and second piston 3 return together to their upper positions with the aid of the unillustrated spring, producing a negative pressure inside the metering chamber A. This negative pressure causes another mass of liquid medicine to flow into the metering chamber A through the nonreturn valve 3d. With the pump mechanism thus constructed, it is possible to eject intermittent sprays of liquid medicine onto an affected part by repeatedly pressing the first piston 2.
The aforementioned conventional pump mechanism is usually mounted to the mouth of the liquid medicine container (not shown) by means of a screw cap 4 and a packing 1f.
One problem of the prior art technology described above is that the amount of liquid medicine ejected through the nozzle chip varies each time the first piston 2 is pressed. Since an upper inside surface 2b1 of the bell-shaped portion 2b of the first piston 2 forms a broad horizontal surface which connects to the axial hole 2c, air is likely to be entrapped under the upper inside surface 2b1 and the entrapped air would not easily be released from the internal space of the bell-shaped portion 2b. This is a main reason which causes the amount of liquid medicine measured by the metering chamber A to usually vary each successive press of the first piston 2.